In recovery of oil from oil wells, pumps are used to draw crude oil from the well bore to the surface well head. The crude oil extracted generally consists of a combination of oil, natural gas, grit, wax and water. The pumps generally comprise two types, namely, continuous flow or on-off pumps, and are powered by either electrical or natural gas motors. Upon emerging at the well head, the crude oil is passed via a pipe to separation tanks where the oil is removed from the mixture extracted from the well bore. The oil may also be temporarily stored in the separation tanks.
The maximum obtainable production rate for a well depends on the rate of migration of crude oil from its geological formation to the well bore. The well bore is unique in having both an inflow and an outflow. The inflow represents the quantity of crude oil that a local formation can deliver to the well bore, whereas the outflow (or rate capacity) represents the quantity of crude oil that can be delivered to the surface (or well head). Typically, the quantity of oil that a pump is able to extract from a well bore (or rate capacity) exceeds the rate of flow of the crude oil from the local formation into the well bore. This situation is normally exacerbated with age of the well. Also, the actual flow rate of crude oil into the well bore can deviate significantly at any particular point in time from an average flow rate for that well.
Thus, it may be seen that if the rate capacity of a pump exceeds the rate capacity of the well, the pump is then operating below maximum efficiency. As the cost of operating the pump is relatively high, this reduced efficiency translates into a wasted cost. Furthermore, severe pump degradation may be caused by having a pump operate above the well production rate. Conversely, if the pump rate falls below the wells production rate, oil accumulates in the well bore resulting in an equilibrium established between oil flowing into the well bore from the formation and causing a resultant drop in production. Furthermore, for progressive cavity type pumps or continuous flow pumps, it is necessary to always maintain fluid in the well bore. Thus, control of the pump rate is relatively more critical in this case.
Thus, there exist the need for a method and apparatus to control pump rates in response to changing rates of oil flow. There have been many attempts in the prior art to mitigate some of these problems, and in particular, the reader is referred to the applicant's U.S. Pat. No. 5,525,040 which describes prior art attempts.